Pre-generated cube map

Hi all,
I've been trying to figure out how to get a pre-rendered cube map into a GLSL program. I assume that the cube map sampler is to be used, but I can't find any information on how the cube map texture is organized - is it one of those cross-cube layouts, six separate textures, some tile pattern..?

I'm going to do this for a sort of sky boxing effect () and I need to do it through GLSL, and it needs to be pre-rendered.